refactor social security code

See [SOCIAL-345](https://jira.springsource.org/browse/SOCIAL-345)

Change SocialAuthenticationFilter to extend AbstractAuthenticationProcessingFilter,
and override unsuccessfulAuthentication to handle redirect exception;
Change SocialAuthenticationRedirectException to extend AuthenticationException;
Add Connection<?> to SocialAuthenticationToken;
Update unit test;

Pull request again to view the code changes.